From: Roger Pau Monné <roger.pau@citrix.com>
Date: Wed, 26 Feb 2020 09:55:22 +0000 (+0100)
Subject: smp: convert cpu_hotplug_begin into a blocking lock acquisition
X-Git-Tag: archive/raspbian/4.14.0+80-gd101b417b7-1+rpi1^2~63^2~632
X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=5b74f5e42884a01646e9ecf12d402ca20d3ddf7a;p=xen.git

smp: convert cpu_hotplug_begin into a blocking lock acquisition

Don't allow cpu_hotplug_begin to fail by converting the trylock into a
blocking lock acquisition. Write users of the cpu_add_remove_lock are
limited to CPU plug/unplug operations, and cannot deadlock between
themselves or other users taking the lock in read mode as
cpu_add_remove_lock is always locked with interrupts enabled. There
are also no other locks taken during the plug/unplug operations.

The exclusive lock usage in register_cpu_notifier is also converted
into a blocking lock acquisition, as it was previously not allowed to
fail anyway.

This is meaningful when running Xen in shim mode, since VCPU_{up/down}
hypercalls use cpu hotplug/unplug operations in the background, and
hence failing to take the lock results in VPCU_{up/down} failing with
-EBUSY, which most users are not prepared to handle.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
---

diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index e83e4564a4..ad49f2dcd7 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -1255,8 +1255,7 @@ int cpu_add(uint32_t apic_id, uint32_t acpi_id, uint32_t pxm)
          (pxm >= 256) )
         return -EINVAL;
 
-    if ( !cpu_hotplug_begin() )
-        return -EBUSY;
+    cpu_hotplug_begin();
 
     /* Detect if the cpu has been added before */
     if ( x86_acpiid_to_apicid[acpi_id] != BAD_APICID )
diff --git a/xen/common/cpu.c b/xen/common/cpu.c
index 0d7a10878c..31953f32e4 100644
--- a/xen/common/cpu.c
+++ b/xen/common/cpu.c
@@ -51,9 +51,9 @@ void put_cpu_maps(void)
     read_unlock(&cpu_add_remove_lock);
 }
 
-bool cpu_hotplug_begin(void)
+void cpu_hotplug_begin(void)
 {
-    return write_trylock(&cpu_add_remove_lock);
+    write_lock(&cpu_add_remove_lock);
 }
 
 void cpu_hotplug_done(void)
@@ -65,8 +65,7 @@ static NOTIFIER_HEAD(cpu_chain);
 
 void __init register_cpu_notifier(struct notifier_block *nb)
 {
-    if ( !write_trylock(&cpu_add_remove_lock) )
-        BUG(); /* Should never fail as we are called only during boot. */
+    write_lock(&cpu_add_remove_lock);
     notifier_chain_register(&cpu_chain, nb);
     write_unlock(&cpu_add_remove_lock);
 }
@@ -100,8 +99,7 @@ int cpu_down(unsigned int cpu)
     int err;
     struct notifier_block *nb = NULL;
 
-    if ( !cpu_hotplug_begin() )
-        return -EBUSY;
+    cpu_hotplug_begin();
 
     err = -EINVAL;
     if ( (cpu >= nr_cpu_ids) || (cpu == 0) )
@@ -142,8 +140,7 @@ int cpu_up(unsigned int cpu)
     int err;
     struct notifier_block *nb = NULL;
 
-    if ( !cpu_hotplug_begin() )
-        return -EBUSY;
+    cpu_hotplug_begin();
 
     err = -EINVAL;
     if ( (cpu >= nr_cpu_ids) || !cpu_present(cpu) )
diff --git a/xen/include/xen/cpu.h b/xen/include/xen/cpu.h
index e49172f64c..e8eeb217a0 100644
--- a/xen/include/xen/cpu.h
+++ b/xen/include/xen/cpu.h
@@ -10,7 +10,7 @@ bool get_cpu_maps(void);
 void put_cpu_maps(void);
 
 /* Safely perform CPU hotplug and update cpu_online_map, etc. */
-bool cpu_hotplug_begin(void);
+void cpu_hotplug_begin(void);
 void cpu_hotplug_done(void);
 
 /* Receive notification of CPU hotplug events. */